1. Field of the Invention
The present invention relates to a wave guide (or waveguide) and a thermally-assisted magnetic recording element using this waveguide.
2. Description of the Related Art
Recently, in a magnetic recording device, such as a hard disk device, performance improvement of a thin film magnetic head and a magnetic recording medium is in demand in association with high recording density. As the thin film magnetic head, a combination type thin film magnetic head is widely used, where a reproducing head having a magneto resistive effect element (hereafter, also referred to as magneto resistive (MR) element) for reading, and a recording head having an inducible electromagnetic conversion element (magnetic recording element) for writing are laminated onto a substrate. In a hard disk device, the thin film magnetic head is placed in a slider that flies slightly above the surface of the magnetic recording medium.
A magnetic recording medium is a discontinuous medium where magnetic micro particles are assembled, and each magnetic micro particle has a single-domain structure. In this magnetic recording medium, one recording bit is composed of a plurality of magnetic micro particles. In order to enhance the recording density, asperity of the boundary between adjacent recording bits has to be small, which means that the micro particles have to be small. However, if the magnetic micro particles are decreased in size, thermal stability of magnetization of the magnetic micro particles is reduced. In order to solve this problem, it is effective to increase anisotropic energy of the magnetic micro particles. However, if the anisotropic energy of the magnetic micro particles is increased, coercive force of the magnetic recording medium becomes great and it becomes difficult to record information in the existing magnetic head. Such a trilemma exists in conventional magnetic recording, which is a great obstacle to increasing the recording density.
As a method for solving this problem, the method of so-called thermal assisted magnetic recording (or thermally-assisted magnetic recording) is proposed. In this method, a magnetic recording medium having great coercive force is used, and a magnetic field and heat are simultaneously applied to a portion where information is recorded in the magnetic recording medium. This causes a rise of the temperature in the portion where the information is recorded and a reduction of the coercive force, and then information is recorded.
In the thermally assisted magnetic recording, a method using near field light is known as a technique to add heat to a magnetic recording medium. Near field light is a type of so-called electromagnetic field to be formed around a substance. Normal light cannot be tapered (narrowed) to a region that is smaller than the wavelength of the light due to a diffraction limitation. However, irradiation of lights with the same wavelength causes the generation of near field light depending upon the microstructure scale, and enables light to be sharply focused on the order of tens of nm on a minimal region. As a specific method to generate the near field light, a method to use metal, referred to as near field light probe generating a near field light from plasmon excited by the light, or a so-called plasmon antenna, is commonly known.
In the plasmon antenna, the near field light is generated by directly irradiating light, but in this technique, a conversion efficiency of the irradiated light to the near field light is low. A majority of the light energy irradiated to the plasmon antenna is reflected by the surface of the plasmon antenna or converted into thermal energy. Since the size of the plasmon antenna is set at or less than the wavelength of the light, the volume of the plasmon antenna is small. Thus, in the plasmon antenna, the temperature rise in association with the heat generation becomes very great.
Such temperature rise causes the plasmon antenna to expand its volume, and to protrude from a medium opposing surface (or an air bearing surface: ABS), which is a surface facing the magnetic recording medium. The end part positioned on the ABS of the MR element is away from the magnetic recording medium, and as a result, there is the problem that a servo signal recorded in the magnetic recording medium cannot be read at the time of recording movement.
Therefore, a technology where no light is directly irradiated to the plasmon antenna is proposed. For example, technology where light that has propagated through a core of waveguide, such as a fiber optic element, is combined with a plasmon generator via a buffer portion in a surface plasmon polariton mode, and the surface plasmon is excited in the plasmon generator, is disclosed in the specification of U.S. Pat. No. 7,330,404. The plasmon generator has an edge of near-field-generator that is positioned on the ABS and that generates a near field light, and a propagation edge facing the waveguide via a buffer portion. The light propagating through the core is totally reflected by the interface between the core and the buffer portion, on which occasion, light that penetrates to the buffer portion, referred to as evanescent light, is generated. This evanescent light and a collective vibration of electrical charge in the plasmon generator are combined, and the surface plasmon is excited to the plasmon generator. The excited surface plasmon propagates to an edge of near-field-generator along the plasmon generator, and generates the near field light at the edge of near-field-generator. According to this technology, since light that propagates through the waveguide does not directly irradiate the plasmon generator, an excessive rise in temperature of the plasmon generator can be prevented.
In an element using a thin film process, such as a magnetic recording element, the core is formed as a slender member having a rectangular cross section. In this case, the waveguide is equipped with a core having a rectangular cross section, and a cladding (hereinafter referred as a clad) surrounding the core, and is occasionally equipped with a member that is referred to as a spot size converter for tapering a laser light. Further, the combination of the waveguide and the plasmon generator is referred to as a near field generator. In a core having the rectangular cross section, it is known that a laser light propagates either in a transverse-electric (TE) mode or a transverse-magnetic (TM) mode. The TE mode is a mode where an electric field component in the core thickness direction becomes 0, and the TM mode is a mode where an electric field component in the core width direction becomes 0. In the TE mode, the electric field component oscillates in the core width direction (the direction indicated with a dashed arrow in FIG. 4A). In the TE mode, the electric field component oscillates in the core thickness direction (the direction indicated with a solid arrow in FIG. 4A).
In order to excite the surface plasmon in the plasmon generator, it is necessary that a laser light exists in the TM mode within the core. In order to excite the surface plasmon, it is desirable that a laser light exists within the waveguide in a specific mode out of the TM mode, and specifically in a mode where only one portion, where the light intensity becomes maximal, exists on the core cross section perpendicular to a propagation direction of the laser light (hereafter, such wave guiding mode is referred to as a fundamental mode) within the waveguide. In the meantime, in order to excite the surface plasmon, it is necessary that the core has a refractive index with a predetermined value or more. However, if the refractive index of the core is increased, a higher order TM mode occurs within the core. Herein, the higher order TM mode means a mode where two or more portions, where the light intensity is maximal, exist on the core cross section (specifically, a mode where two or more portions exist in the x direction or the z direction or both in FIG. 4A), and includes all wave guiding modes except for a fundamental mode. It is also known that the higher order TM mode easily occurs when the core thickness is increased. The higher order TM mode does not only effectively contribute to the excitation of the surface plasmon, but there is also a possibility to inhibit the excitation of the surface plasmon in the fundamental mode.
The objective of the present invention is to provide a waveguide enabling attenuation of a higher order TM mode in a waveguide where a laser light including the higher order TM mode propagates. Further, the objective of the present invention is to provide a thermally-assisted magnetic recording system of magnetic recording element using such a waveguide. In addition, the objective of the present invention is to provide a slider, a head gimbal assembly and a hard disk device using such a magnetic recording element.